The spine apparatus (SA) is a specialization of the smooth endoplasmic reticulum (SER). It is almost always present in dendritic spines that have a perforated synapse, i.e. the mushroom spine or its sessile variety. The SA is typically located in the head of the spine but sometimes can be found in the neck or, in the case of the sessile spine, in the dendritic shaft.
In its most characteristic form, one narrow tubule of the SER turns from the dendritic shaft into the neck of the spine to reach the head and branch into two or more stacked flat cisternae (Fig. 1). The larger the synapse, the more cisternae present. Between the cisternae a dense material is present, called inner dense plates (Figs. 1, 2, 4). A fine filamentous material radiates from these plates to a punctum adherens-like part of the postsynaptic density (PSD) or occasionally, to the punctum adherens which binds an astrocytic process to the dendritic spine. We call this filamentous band an outer dense plate.
|Fig. 1: Diagram of a dendritic spine (ds) containing the spine apparatus. From the inner dense plates localized between the stacked cisterns (yellow) a filamentous material (red) radiates into the punctum adherens-like part of the postsynaptic density (psd). The narrow tubule running through the neck of the spine (n) connects the cisterns with the smooth endoplasmic reticulum (ser) of the dendritic stem (st). An intimate apposition of the reticulum to a mitochondrion (m) is visible in the dendritic stem.|
|Fig. 2: An electron micrograph demonstrating the fine granulofilamentous material (arrow) radiating from the dense plates of the spine apparatus (SA) into the punctum adherens-like part of the postsynaptic density. (Ax) Axon terminal. (Mouse neocortex. Spacek, 1985b, with permission of Springer-Verlag).|
|Fig. 3: Three-dimensional reconstruction of a dendritic spine from the hippocampal stratum radiatum. A perforated excitatory synapse (red) and a macular inhibitory one (blue) are located on the head of the spine. The spine apparatus (yellow) is located in the head and neck of the spine.|